1. Field of the Invention
This invention relates generally to fiber optic communications and, more particularly, to optical switching techniques for fiber optic communications.
2. The Prior Art
The use of fiber optics to convey information is expanding at an ever-increasing rate. And speeds at which information is moving along fiber optics is also increasing. As a consequence, the need for more and faster optical switching is also increasing and is a critical technique in the continued proliferation of fiber optic communications. Currently available optical switching techniques, such as electro-optical waveguide methods, microelectromechanical (MEM) mirror methods, ink jet bulb methods, cannot meet the ever-increasing need for speed by rapidly developing optical communication networks. The most challenging requirements of an optical switch, resulting from the most recent optical network developments include the following.
(a) The switching device must have a sufficiently long service lifetime. For example, taking into account current device fabrication capabilities, the typical service lifetime of MEM devices is but a few million operations. By simply comparing the basic facts that a provisioning switch, a switch used for cross-connection and reconfiguration of different light paths in an optical network, must typically perform tens, if not hundreds, of operations per second and that there are approximately 31.5 million seconds in one year, the service life of current MEM devices means that they are clearly inadequate for use as provisioning switches. Because optical switches are constantly working with frequent operations, and the replacement of a major optical switch in the network is an expensive proposition, a service life of hundreds or thousands of times longer than current MEM devices is appropriate. In order to ensure substantially a longer service life, new materials and processes must be used.
(b) It is highly desirable that an optical switching system be scaleable and have a large potential capacity. For example, an optical switch that has Nxc3x97N=32xc3x9732 ports (connecting a group of 32 optical fibers to another group of 32 optical fibers) is currently considered to be a large switch, and it is expected that N will increase rapidly in the foreseeable future. One problem with the optical switching methods of the prior art is that, in all current switch structures for connecting one group of N optic fibers to another group of N fibers, there must be Nxc3x97N switching cell elements. See, L. Y. Lin et al., Free-Space Micromachined Optical Switches for Optical Networking, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 5, No. 1, pg 4 (1998) and U. S. Pat. No. 6,091,867, issued to Young et al. The Nxc3x97N=N2 rule makes the optical switch structure at least very inefficient for moderate size switches and practically unfeasible for very large optical switches. For continued development, new switching structures must be provided.
(c) The speed of optical switch is another important performance parameter. A minimum switching speed of 10 ms is considered acceptable for a provisioning optical switch. MEM device switches have a speed of few milliseconds. However, as network speeds increase, the need for higher speed switches will become more important.
An object of the present invention is to provide an optical switch structure that can switch a single source signal to multiple destinations.
Another object is to provide an optical switch structure with microsecond response times.
Yet another object is to provide a switch structure that can be used in an optical switching system for cross-connecting multiple optical fibers using only one switch element per fiber.
A further object is to provide a switch structure that can be used in an optical switching system for cross-connecting multiple optical fibers to each other.
The present invention is an optical switching system for use in building optical switching systems that have the capability of providing optical cross-connections among a large number of optical fibers for fiber optic communications. The system employs several optical switching modules, each of which provides a connection between two optical fibers. The switching module comprises (a) a source channel, (b) a transmitting element, (c) a receiving element, and (d) a destination channel.
The source channel is the source of the signal that is being switched to an arbitrarily selected destination channel. It is a single optical fiber of a type used for fiber optic communications. A collimating lens transforms the light energy emitted from the fiber to a collimated light beam, and a connector couples the optical fiber to the collimating lens. The transmitting element includes an initial beam deflector, which produces a small change of the direction of the collimated light beam. A beam deflection amplifier amplifies the deflection of the initially-deflected light beam to produce a fully deflected light beam in the direction of the destination channel. The receiving element ensures that the light beam is correctly aligned for the destination channel. It includes a beam deflection compressor, which decreases the angle of the fully deflected light beam relative to the destination channel, and a beam aligner, which provides a fine direction change for aligning the coarsely aligned beam precisely with the destination channel. The destination channel includes a focusing lens for focusing the aligned light beam to the open end surface of the destination optical fiber.
The present invention contemplates at least four embodiments of the switching module, where the differences between embodiment lies with the transmitting and receiving elements. The first embodiment comprises a pair of focusing lenses and a piezoelectric actuator, where one of the lenses is rigidly bonded to the actuator. When the axes of the two lenses coincide with the axis of a collimated light beam, the light beam is focused at a focal point on the optical axis of the system. When the axis of one lens deviates from the light beam axis by a distance d and the lenses are a distance s=f1+f2 apart, then the light beam becomes collimated again after passing through the second lens, but with a deflection angle xcex1=d/f2.
The beam deflection amplifier multiplies the small initial deflection angle xcex1 by a transfer function F to result in a beam with a full deflection angle Fxcex1. There are four preferred configurations for the beam deflection amplifier. The first is a standard telescope lens system. The second preferred configuration is to use a compound lens system as lens L2 of the initial deflector. The third preferred configuration is the lens system disclosed in the U.S. Pat. No. 6,204,955, entitled APPARATUS FOR DYNAMIC CONTROL OF LIGHT DIRECTION IN A BROAD FIELD OF VIEW. The fourth preferred configuration is the lens system disclosed in the U.S. Pat. No. 6,295,171 entitled PIEZOELECTRIC LIGHT BEAM DEFLECTOR.
In the receiving element, the beam deflection compressor has a transfer function G, where xe2x88x921 less than G less than 1, which when applied to the output of the beam deflection compressor results in a beam with a deflection angle of GFxcex1. A preferred embodiment of the receiving element is simply the transmitting element used in reverse. When G=1/F, the light beam output from the beam deflection compressor has a deflection angle xcex1. Likewise, a preferred embodiment of the beam aligner is the initial beam deflector used in reverse.
The second embodiment of the switching module is the same as the first embodiment except that the positive lens L2 is replaced by a negative lens L2N. The formula for determining the deflection angle is the same as that of the first module embodiment, except that f2 less than 0.
In the third embodiment, the small deflection angle change is produced by a separate tilting mirror that is controlled by a piezoelectric actuator.
In the fourth embodiment, the initial beam deflector is another electrically-controlled beam deflector such as acousto-optical light beam deflectors, surface acousto-optical wave (SAW) deflectors, electro-optical light beam deflectors, electrically-controlled light grating deflectors, liquid crystal light beam deflectors, light grating valve devices, etc. These light beam deflectors are advantageous in many cases because they have no mechanical moving parts.
The first system configuration connects a single channel A to one of a number of B channels. The system consists of a single transmitting element and a number of receiving elements, one for each B channel. The group B channel to which the source signal is directed is determined by the deflection angle of the transmitting element. Note that, because all of the optical components of the system are bi-directional, the switch will operate in either direction.
The second switching system configuration connects a group of M channels (group A) with another group (group B) of N channels. Again, since all components are bi-directional, signals can travel in both directions. This configuration illustrates one of the main advantages of the present invention over the switching systems of the prior art: to cross-connect a group of M channels to a group of N channels, only M+N switch cells are needed, rather than the Mxc3x97N switches that are needed by prior art switching systems.
The third switching system configuration can cross-connect N channels in an arbitrary manner. When the channels are positioned with a symmetric geometry, each channel can transmit and/or receive a signal from any other channel through a pair of single switch cells and a mirror. The mirrors are fixed relative to the other system components and adjusted to a specific orientation so that, when no switching signals are applied to any of the channels, each channel is statically connected to one and only one other channel. Thus, even with no switching signals present, all channels are mutually connected.
Other objects of the present invention will become apparent in light of the following drawings and detailed description of the invention.